A triacetate film has been used as a base material for a photo film. This triacetate film involves safety and environmental problems because an organic solvent is used in the production process of the film. It also has such a defect that it has limits in its mechanical strength and dimensional stability. Therefore, a polyethylene terephthalate film has started to be used as a substitute material in some industries. However, when it is stored in a rolled state, its curling remains persistently and it is difficult to remove it. Therefore, such a film is inferior in its handling properties after development, thereby making it difficult to use it as a photo film which is used as a roll film.
As a technology for improving curling resistance, JP-A 53-146773, JP-A 1-244446 and the like propose modified polyethylene terephthalate films having improved water-vapor permeation, improved moisture content and the like. Although the effect of suppressing curling is recognized in these films, they have such defects as a reduction in dimensional stability caused by moisture absorption, an increase in the deformation of an edge portion of a film caused by a fall in glass transition temperature and the like. Therefore, these films are unsatisfactory as a photo film.
Further, in recent years, photo films of higher quality have been demanded along with an increase in the winding speed of a film at the time of shooting and a reduction in the size of a photographing device (camera), in the case of a roll film. It has been desired that the film show excellent curling resistance even when rolled to a small diameter and that it have high mechanical strength and dimensional stability even when reduced in thickness. These requirements can be satisfied by neither the conventional triacetate film nor the above modified polyethylene terephthalate films. Therefore, a photo film having excellent properties has been demanded.
To meet these demands, the application of a polyethylene naphthalate (PEN) film to a photo film is proposed by JP-B 48-40414, JP-A 50-109715 and the like. Since the films disclosed by the above publications have high mechanical strength and dimensional stability, they show excellent adaptability to a reduction in thickness and are satisfactory to some extent in terms of curling resistance when rolled to a small diameter. However, they involves another problem that delamination (interlaminar peeling) easily occurs in the films, particularly when the film is perforated in the production of a roll film. In this case, they have a problem with their use as a base film for a photo film because a delaminated portion is whitened.
JP-A 8-104742 discloses a modified polyester having a glass transition temperature (Tg) higher than polyethylene naphthalate (PEN) and a film thereof. It also teaches that the modified polyester film is used for photographic application. In this proposal, modified PEN comprising, as a copolymer component, an ester-forming derivative having a fluorene group, phenylindane group, spiroglycol group or diphenylsulfone group is given as the modified polyester having a higher Tg than PEN. Specific examples disclosed in the above publication show that modified PEN comprising, as a copolymer component, at least 17 mol % of a diol containing a fluorene group (Example 6) has a Tg of 126.degree. C. and that modified PEN comprising, as a copolymer component, 11 mol % of this diol (Comparative Example 6) has a low Tg and a film made of the modified PEN has low curling resistance.
The invention disclosed in the above publication is intended to obtain a modified polyester having Tg higher than that (Tg=122.degree. C.) of PEN. Therefore, when the copolymer component is a diol containing a fluorene group, it is used in an amount of 17 mol % or more. A film made of this modified polyester is unsatisfactory in terms of Young's moduli and causes troubles in the handling and other treatments of the film.